This paper investigates the use of multi-word sequences in two important university registers: classroom teaching and textbooks. Following Biber et al. (1999), we take a frequency driven approach to the identification of multi-word sequences, referred to as "lexical bundles". We compare the lexical bundles in classroom teaching and textbooks to those found in our previous research on conversation and academic prose. Structural patterns are described first, and then we present a functional taxonomy, including stance expressions, discourse organizers, and referential expressions. The use of lexical bundles in classroom teaching turns out to be especially surprising, both in frequency and in function. Classroom teaching uses more stance and discourse organizing bundles than conversation does, but at the same time, classroom teaching uses more referential bundles than academic prose. The analysis indicates that lexical bundles--the most frequent sequences of words in a register--are a unique linguistic construct. Lexical bundles are usually not complete grammatical structures nor are they idiomatic, but they function as basic building blocks of discourse. In the conclusion, we discuss the implications of our study for the theoretical status of lexical bundles.